Gamma-radiation (“γ-radiation”) imaging is a well established imaging technique in the fields of nuclear medicine and astrophysics. In recent years, the outstanding ability of γ-radiation detectors to image and characterize any known as well as unknown γ-radiation source has been applied to other fields, such as biomedical research and investigation of suspicious target materials at airports. γ-radiation imaging can be used to gain information from concealed targets, such as explosives, drug- or nuclear-based contraband, and the like, or malignant tumors in the human body by accurately detecting the emitted signal from the target material.
One technique known for obtaining information from concealed targets via γ-radiation imaging utilizes neutron activation. Neutron activation is a process in which neutron radiation induces a radioactivity in the target material. Neutron activation is performed by irradiating neutrons into an area of the target material of interest, thereby exciting atomic nuclei within the target material. The excited nuclei are subsequently relaxed by emitting subatomic particles and/or γ-radiation. Appropriate detection of the γ-radiation and analysis of the detected spectrum facilitates the identification of a particular substance within the designated location of the target material.
Scintillation detectors such as sodium iodide (NaI) scintillators coupled with a photomultiplier tube (PMT) have been used for detecting γ-radiation induced from the target material by neutron activation. While scintillation detectors are known to have fast response times (in the order of nanoseconds) with relatively simple structure, they exhibit poor energy resolution of γ-radiation thereby causing, inter alia, a loss of information. Because γ emissions from different isotopes of the target material that have similar γ energy spectrums cannot be properly separated, there can be a loss of information.
Solid-state detectors such as silicon detectors, silicon-germanium detectors and high purity germanium detectors (HPGe) have been used in detecting γ-radiation of the target material irradiated by fast neutrons. Solid-state detectors include a substrate of semiconductor materials such as silicon, silicon-germanium, high purity germanium (HPGe) or the like, where each is placed between collecting electrodes. Semiconductor detectors are known to have a good energy resolution of the order of 0.1-0.3% for the HPGe detectors as compared to the scintillation detectors that provide an energy resolution on the order of 10%. The higher resolution of the semiconductor detectors are due primarily to the interaction of γ-radiation with the semiconductor material, which produces a charge that is directly collected by the electrodes. Semiconductor detectors, however, tend to have a relatively slow response time, of about 200 ns for the HPGe detectors, as compared to the scintillation detectors, which have a response time of about 1-2 ns. The slower response time of the semiconductor detectors are due primarily to the lag time of electrons drifting through the bulk thickness of the semiconducting material, before generating a detectable signal. As a result, the signals from the semiconductor detectors are usually shaped and integrated over a period of time of up to several microseconds.
Integrating the signal of semiconductor detectors over several microseconds tend to add high background levels of γ-radiation signals to the detected spectrum, thereby causing severe background noise. For example, additive γ-radiation signals coming from uncorrelated scattering events that are not directly related to the interaction of the neutrons with the target's nuclei are often generated during the relatively long integration time. Since the signal-to-background ratio (SBR) determines the quality of the detected spectrum, high background radiation signal levels are undesirable in γ-radiation imaging.
In view of these foregoing and other considerations, there is a need to develop novel imaging mechanisms that can provide high-resolution spectra with fast response times and improved signal-to-background ratios.